The method of reflection seismology is an important tool used to image or determine properties of the earth. The method generally includes a source, or distribution of sources, usually on or near the earth's surface that excites a seismic wave into the earth; and a receiver, or distribution of receivers, that records the wave that is reflected (or scattered) back from the earth. Many different types, or modes, of seismic waves are known. The compressional wave, or P-wave, is the most widely used mode. The shear wave, or S-wave, is another mode that has proven to have some utility under certain circumstances. A source may excite one or both modes, and a receiver may record one or both modes.
The most frequent implementation of the reflection seismology method exploits the P-wave. A P-wave is excited by the source and propagates into the earth where it is reflected (or scattered) back from the earth and recorded by receivers. This is known as PP reflection seismology. Another implementation of the reflection seismology method exploits the S-wave; an S-wave is excited by the source and propagates into the earth where it is reflected (or scattered) back from the earth and recorded by receivers. This is known as SS reflection seismology. Both the PP and SS reflection seismology methods are referred to as “common mode” methods because the wave mode incident into the earth from the source is the same as the wave mode reflected (or scattered) back from the earth.
The practice of seismic survey design incorporates many methods and procedures to estimate requirements for seismic survey acquisition parameters so that specific technical objectives may be achieved with the acquired seismic data. One such objective is seismic image resolution. With regard to common mode seismic surveys, methods and procedures exist to determine required acquisition parameters to achieve desired seismic image resolution. Many aspects of these methods and procedures are discussed in the geophysical and imaging literature and have been wholly or partly used in various forms in the practice of seismic survey design for many years. Though some unresolved issues remain, the practice of designing common mode seismic surveys by one or another of these methods and procedures to achieve specified seismic image resolution is well established. See for example, “Planning Land 3-D Seismic Surveys,” by Cordsen, et al., Society of Exploration Geophysicists (2000).
In recent years, some attention has turned to seismic surveys which are not common mode but are, rather, “converted mode,” sometimes referred to as “converted wave.” The converted mode seismic survey differs from the common mode seismic survey in that the wave mode incident into the earth from the source is different from the wave mode reflected (or scattered) back from the earth. An example is PS reflection seismology, where a P-wave is excited by the source and propagates into the earth, and an S-wave is reflected (or scattered) back from the earth and recorded by receivers. Another example is SP reflection seismology, where an S wave is excited by the source and propagates into the earth, and a P-wave is reflected (or scattered) back from the earth and recorded by receivers. The characteristics of converted wave reflection seismology are substantially different from common mode reflection seismology. Methods and procedures used in designing common mode reflection seismology surveys are not directly applicable to designing converted mode reflection seismology surveys. Consequently, though the practice of design of common mode seismic surveys to achieve specified seismic image resolution is well established, the same is not true for converted mode seismic surveys.
Converted mode surveys are sometimes pursued when it is known that a compressional wave survey will not produce the required information. This has been demonstrated, for example, in some marine circumstances where shallow gas charged sections prevent compressional wave imaging of deeper sections; and has been demonstrated to some degree in cases where the P-wave impedance of the target reservoir is very low and undetectable relative to S-wave impedance. There have been some reports from recent research that converted mode surveys may provide useful added information, generally, and should therefore be run more often in conjunction with conventional surveys. Of the two types of converted wave surveys, PS is the most discussed and practiced to date. The SP survey has been problematic to implement in the marine case. Publications that discuss design of converted mode seismic surveys include:                Gijs J. O. Vermeer, “Converted waves: properties and 3D survey design,” 69th Annual International Meeting of the Society of Exploration Geophysicists, Expanded Abstracts, SACQ22.6, pp. 645-648 (1999);        Gijs J. O. Vermeer, “3-D Seismic Survey Design” (especially Chapters 6 and 8), Society of Exploration Geophysicists (2002);        B. Nolte, J. Etgen and R. Read, “3D OBS (Ocean-Bottom Seismic) survey design: estimating lateral resolution and acquisition footprint,” EAGE 66th Conference & Exhibition, P197, (2004);        Peter W. Carey and Don C. Lawton, “Bandlimited design and stacking of P-P and P-S surveys,” 73rd Annual International Meeting of the Society of Exploration Geophysicists, Expanded Abstracts, MC 3-8, pp. 842-845 (2003).        C. Jason Criss and Jim Musser, “Ocean-bottom survey acquisition design for 3D converted waves,” Offshore Int. 63, 129-130, (2003).        R. Garotta and P. Y. Granger, “Some requirements for PS-mode acquisition,” The Leading Edge 22, 106-112 (2003);        Robert R. Stewart et al., “Converted wave seismic exploration: Methods,” Geophysics 67, 1348-1363 (2002);        Jurgen Hoffman, “Illumination, resolution, and image quality of PP- and PS-waves for survey planning,” The Leading Edge 20, 1008-1014 (2001); and        Don Lawton, “Converted-wave 3-D surveys: Design strategies and pitfalls,” 22nd Annual Canadian Society of Exploration Geophysicists National Convention Abstracts, pp. 69-70 (1995).        
What is needed is a method for designing a converted mode seismic survey that is as developed and complete as are current methods for designing common mode seismic surveys, and that yields a survey design that satisfies pre-selected resolution criteria at target depth. The present method satisfies this need.